34 research outputs found
A ghost and a naked singularity; facing our demons
We encounter these demons on the path towards a UV complete QFT of gravity
and a horizonless replacement for black holes. The fate of the ghost is
discussed in the strong coupling version of classically-scale-invariant
quadratic gravity. Rather than a propagating ghost, the full graviton
propagator ends up with a slight acausal behavior. We then turn to the 2-2-hole
solutions appearing in a classical approximation of the gravity theory. We
present a new solution that is sourced by a relativistic gas, where a benign
timelike singularity is shrouded by a fireball. Calculating the standard
entropy gives an area law, and the entropy of a 2-2-hole can exceed that of the
same size black hole. Finally, an observational consequence of 2-2-holes takes
the form of gravitational wave echoes that can be generated by a newly formed
2-2-hole after a 2-2-hole merger. LIGO is sensitive to such a signal. We update
the evidence from our previous search with five additional events reported more
recently by LIGO.Comment: 13 pages, 23 figures, talk given at CERN workshop "Scale Invariance
in Particle Physics and Cosmology", Jan 28 - Feb 1, 201
A QCD analogy for quantum gravity
Quadratic gravity presents us with a renormalizable, asymptotically free
theory of quantum gravity. When its couplings grow strong at some scale, as in
QCD, then this strong scale sets the Planck mass. QCD has a gluon that does not
appear in the physical spectrum. Quadratic gravity has a spin-2 ghost that we
conjecture does not appear in the physical spectrum. We discuss how the QCD
analogy leads to this conjecture and to the possible emergence of general
relativity. Certain aspects of the QCD path integral and its measure are also
similar for quadratic gravity. With the addition of the Einstein-Hilbert term,
quadratic gravity has a dimensionful parameter that seems to control a quantum
phase transition and the size of a mass gap in the strong phase.Comment: 27 pages, 3 figures, matches published versio
Nuclear-size effects and a numerical approach to the Dirac equation
Due to some current interest in this subject we have produced this note.
There is no claim to anything new, except possibly to show that a direct
numerical approach is quite simple and instructive. For comparison purposes we
include a section on the Coulomb Klein-Gordon equation.Comment: 11 pages, 5 figure
Gravitational Wave "Echo" Spectra
Exotic compact objects may resemble black holes very closely while remaining
horizonless. They may be distinguished from black holes because they
effectively give rise to a resonant cavity for the propagation of low frequency
gravity waves. In a Green's function approach, the resonance structure appears
in a transfer function. The transfer function in turn is modulated by an
initial-condition-dependent source integral to obtain the observed spectrum. We
find that the source integral displays universal factors that tend to enhance
low and negative frequencies, and this increases the complexity of the
waveforms in the time domain. These waveforms also display a significant
sensitivity to initial conditions. For these reasons a standard matched-filter
search strategy is difficult. In contrast, the sharp and evenly spaced
resonance spectrum presents a much more robust signal to target. It persists
even in the absence of simple echoes. We also describe an additional
two-component structure of this resonance pattern.Comment: 17 pages, 6 figure
Not quite black holes at LIGO
We provide more evidence of not quite black holes at LIGO. We update and
streamline our previous search strategy and apply it to the ten black hole
merger events and the one neutron star merger event. The strategy is aimed at
the evenly spaced resonance spectrum expected from not quite black holes, given
that at low frequencies the radial wave equation describes the modes of a
stretched 1D cavity. We describe various indications of the self-consistency of
the apparent signals across all events in the context of a simple theoretical
model. The merger with the largest final mass, spin and redshift, GW170729,
provides additional interesting support.Comment: 20 pages, 16 figures, published versio
Ultra-Planckian scattering from a QFT for gravity
We show that astonishing cancellations take place in the calculation of
high-energy scattering cross-sections in quantum quadratic gravity, a quantum
field theory for gravity. Tree-level differential cross-sections, either
exclusive or minimally inclusive, behave as , as desired for a
well-behaved UV completion. We argue that ultra-Planckian scattering involves
gravitational parton showers, both in the initial and final states.
Differential cross sections are calculated for the various hard scattering
processes involving the various spin states of the massless and massive
gravi-particles. We also discuss some of the simpler amplitudes. Unitarity
without positivity is the key property of the perturbative theory.Comment: 23 pages, no figure
Gravitational wave echoes through new windows
There has been a striking realization that physics resolving the black hole
information paradox could imply postmerger gravitational wave echoes. We here
report on evidence for echoes from the LIGO compact binary merger events,
GW151226, GW170104, GW170608, GW170814, as well as the neutron star merger
GW170817. There is a signal for each event with a -value of order 1% or
sometimes significantly less. Our study begins with the comparison of echoes
from a variety of horizonless exotic compact objects. Next we investigate the
effects of spin. The identification of the more generic features of echoes then
leads to the development of relatively simple windowing methods, in both time
and frequency space, to extract a signal from noise. The time delay between
echoes is inversely related to the spacing between the spectral resonances, and
it is advantageous to look directly for this resonance structure. We find time
delays for the first four events that are consistent with a simple model that
accounts for mass and spin of the final object, while for the neutron star
merger the final mass and spin are constrained.Comment: 37 pages, 19 figures, matches version to be published in PR
Searches for the of a fourth family
We study the detection of the of a fourth family during the early
running of LHC with 7 TeV collision energy and 1 fb integrated
luminosity. By use of a neural network we show that it is feasible to search
for the even with a mass close to the unitarity upper bound, which is in
the 500 to 600 GeV range. We also present results for the Tevatron with . In both cases the search for a fourth family quark
doublet can be significantly enhanced if one incorporates the contribution that
the can make to a -like signal. Thus the bound on the mass of a
degenerate quark doublet should be stronger than the bounds obtained by
treating and in isolation.Comment: 10 pages, 4 eps figures, 5 Tables, publication versio
Not quite black holes as dark matter
Primordial black holes that survive until the present have been considered as
a dark matter candidate. In this paper we argue that primordial 2-2-hole
remnants provide a more promising and testable option. 2-2-holes arise in
quadratic gravity as a new family of classical solutions for ultracompact
matter distributions and they possess the black hole exterior without an event
horizon. They may serve as the endpoint of gravitational collapse, providing a
resolution for the information loss problem. Intriguing thermodynamic behavior
is found for these objects when sourced by a thermal gas. A large 2-2-hole
radiates with a Hawking-like temperature and exhibits an entropy-area law. At a
late stage, the evaporation slows down and essentially stops as the mass
asymptotically approaches a minimal value. This remnant mass is determined by a
fundamental scale in quadratic gravity. We study the cosmological and
astrophysical implications of having these remnants as dark matter and derive
the corresponding constraints. A distinctive phenomenon associated with remnant
mergers occurs, predicting fluxes of high-energy astrophysical particles due to
the spectacular evaporation of the merger product. Measurements of high-energy
photon and neutrino fluxes could possibly bound the remnant mass to be not far
above the Planck mass. Early-universe physics, on the other hand, requires that
2-2-holes quickly evolve into the remnant state after formation, putting an
upper bound on the formation mass.Comment: 33 pages, 10 figure
Unruh-DeWitt Detector Differentiation of Black Holes and Exotic Compact Objects
We study the response of a static Unruh-DeWitt detector outside an exotic
compact object (ECO) with a general reflective boundary condition in 3+1
dimensions. The horizonless ECO, whose boundary is extremely close to the
would-be event horizon, acts as a black hole mimicker. We find that the
response rate is notably distinct from the black hole case, even when the ECO
boundary is perfectly absorbing. For a (partially) reflective ECO boundary, we
find resonance structures in the response rate that depend on the different
locations of the ECO boundary and those of the detector. We provide a detailed
analysis in connection with the ECO's vacuum mode structure and transfer
function.Comment: 18 pages, 22 figure